The present invention relates to the field of impulse ink jet printing, and more particularly to the field of impulse ink jet inks which are used in printers which contain drop-on-demand impulse ink jet heads. Such printers are normally used with a number of substrates the most common of which are papers of various porosities.
In order to achieve an acceptable print quality, it is known to carefully select the substrate materials to match them to the particular characteristics of the ink which is being jetted. Such careful selection is possible in certain environments, such as in large scale package printing operations, but is not preferred in many environments where there is less control over the selection of the substrate for use with the particular ink. Accordingly, a need exists for impulse ink jet inks which are useful with a wide range of substrates.
Many impulse ink jet inks known to the art are liquid inks. These inks have many advantages. They are easy to store and ship (provided the colorant is readily soluble in the vehicle system), and normally exhibit low viscosities at room temperature, making them quite easy to jet. On the other hand, liquid inks tend to penetrate and spread when jetted onto a porous substrate (such as high rag content bond paper). As a result, it is difficult to achieve letter quality print with liquid impulse ink jet inks without using special substrates. Often the print has a washed out and diffuse appearance. At the present time, liquid impulse ink jet printing systems have achieved only limited acceptance in the general office environment.
It is also known to use inks which are solid at room temperature but which are readily melted for jetting. Many of these inks are described in the above-referenced patent applications and patents. Hot melt inks are referred to, for example, in U.S. Pat. No. 4,490,731 (Vaught). See also U.S. Pat. No. 3,653,932 (Berry); U.S. Pat. No. 3,715,219; and U.S. Pat. No. 4,390,369 (Merritt). These "hot-melt" inks normally comprise vehicles, such as natural waxes, resins and/or long chain fatty acids, esters or alcohols which melt when the ink is heated to jetting temperatures. Upon jetting, heated droplets impact the substrate and immediately freeze on the substrate surface. This phenomenon is advantageous in several respects, in that dark, sharply defined print may be produced. This print may be slightly raised, suggesting that the print is engraved. Since the ink is solid at room temperature, during storage and shipment colorant systems have less of a tendency to separate out of the ink. This has facilitated the use of various colorant systems, such as certain pigment based systems, which would not normally have been used in liquid inks.
There are also known disadvantages to many hot-melt phase change inks. There are certain conditions of use and/or storage where ambient temperatures to which the printed materials are exposed far exceed room temperatures. Papers stored in the trunk of an automobile on a sunny day may, for example, far exceed 100.degree. F. (38.degree. C.). Under such conditions, phase change inks may remelt, causing unwanted adherence to adjacent pages in regions where the original print was raised above the surface of the substrate. Because they are solid at room temperature and tend to be located at the surface of the substrate, they are also prone to burnishing, cracking or flaking. Proper use of hot melt inks therefore requires proper selection of the ink for the intended conditions of use. It is advantageous to select the substrate and jetting conditions to obtain some degree of ink penetration while achieving desirable print characteristics.
Notwithstanding the substantial advances achieved in the development of ink jet inks for use with drop on demand printers, a need still exists for inks which will produce dark, clearly defined print on a wide range of substrates and which will not crack, smear, flake off or burnish.
The present invention provides a novel ink jet printing methods which utilizes semi-solid inks. These inks are to be distinguished from those which may contain one or more components which may be semi-solid at room temperature but which, in the proportions or combinations used in the subject inks, do not result in inks which are semi-solid at room temperature. For example, U.S. Pat. No. 4,357,431 discloses an ink comprising a polymeric substance selected from a group consisting of polyisobutylene and ethylene-propylene copolymer, a wax which is solid or semi-solid at room temperature, pigment particles and a low-boiling solvent having a boiling point lower that 200 degrees C. The resulting composition, which is intended for use as an ink in ball point pens, has a relatively low viscosity ("preferably in a range of 500 to 1500 poises").